1. Field of the Invention
This invention relates generally to a vehicle acceleration control system and specifically to a traction control system for preventing or limiting the slip of vehicle wheels to enable a smooth start and maximize acceleration of a vehicle.
2. Description of the Prior Art
Some known vehicle slip control systems perform traction control for adjusting the rotation of vehicle driving wheels to maximize friction between the driving wheels and the road surfaces and thereby to prevent or control slip of the driving wheels during acceleration of the vehicle. This slip control improves the vehicle running stability and the vehicle acceleration ability.
Japanese published examined patent application No. 54-42077 discloses such a vehicle slip control system. In the system disclosed in this published application, when the degree of slip of the vehicle driving wheels exceeds a reference value, misfire a of a vehicle powering engine is forcedly induced or the fuel supply to the engine is cut off to reduce the engine power output to control rotation of the driving wheels. The reference value of slip is periodically determined and updated in accordance with deduced conditions of road surfaces. The periodic updating of the slip reference value can deteriorate the response characteristics of the slip control. The deteriorated response characteristics of the slip control sometimes cause an unstable acceleration of the vehicle. In addition, the misfire generally increases polluting emission from the engine. Furthermore, the misfire and the fuel cut off may result in a sudden change of the operating state of the engine which can cause unacceptable shock or vibration of the engine.
Japanese published unexamined patent application No. 58-38347 discloses a vehicular engine control system which performs vehicle wheel slip control. In the system disclosed in this published application, the speed of vehicle driving wheels is compared to the speed of the vehicle idler or non-driving wheels to determine the slip ratio of the driving wheels. When the difference between the driving wheel speed and the idler wheel speed exceeds a reference value, the torque transmitted to the driving wheels from an engine is reduced by cutting off the fuel supply to the engine and by forcedly changing the gear ratio in a power transmission, and the sudden reduction of the fuel supply to the engine can increase polluting emission from the engine. The forced change of the transmission gear ratio impair fuel economy of the engine. As described previously, the fuel cut off results in a sudden change of the operating state of the engine which can cause unacceptable shock or vibration of the engine.